1. Field of the Invention
The present invention relates to a wavelength-division-multiplexed optical source and a passive optical network system employing the same, and more particularly, to a wavelength-division-multiplexed optical source for providing data services and broadcasting services, and to a passive optical network system employing the same.
2. Description of the Related Art
A Wavelength-Division-Multiplexed Passive Optical Network (WDM-PON) provides broadband communication services at very high speed by using intrinsic wavelengths assigned to each subscriber. Therefore, the WDM-PON can keep a communication secret with certainty and can easily accommodate an increase in communication capacity as well as a special communication service requested by a subscriber. In particular, the WDM-PON can be reconfigured for new subscriber terminals just by adding intrinsic wavelengths to be assigned to each additional terminal. Advantageously, the WDM-PON can thus easily be made to accommodate extra subscriber terminals.
However, a central office (CO) and subscriber terminals of which the WDM-PON is comprised must have at least one optical source with an assigned oscillation wavelength and at least one wavelength-stabilizing circuit for stabilizing the wavelength of the optical source, which imposes a high cost burden on the subscribers to the WDM-PON. The WDM-PON has not yet been put to practical use for this reason in spite of its many advantages. There accordingly exists a need for an economical optical source in order to put the WDM-PON to practical use.
Implementation of a broadcasting service through a WDM-PON, instead of through another hybrid coaxial network (HFC network) as is currently conventional, would also advantageously reduce cost. Therefore, a number of studies for utilizing a WDM-PON for a broadcasting service are vigorously being pursued, and a variety of methods for providing a broadcasting service have been proposed. Examples of methods for providing a broadcasting service include through a distributed feedback laser (DFB laser), through a distributed feedback laser array (DFB laser array), and through a spectrum-sliced light source. The characteristics of each method are as follows.
The broadcasting service provision method using a DFB laser directly modulates a distributed feedback laser in accordance with broadcasting service signals, amplifies the modulated signals through an optical amplifier, and outputs the amplified signals through a power splitting optical link to provide the broadcasting service to each subscriber terminal. The power splitting optical link is provided with a special link so as to be differentiated from the optical link of WDM for data service.
This method complicates the manufacturing procedure and requires the use of high-priced elements which are necessary to provide accurate wavelength selectivity and wavelength stability of a WDM optical source. The method further requires a special power splitting optical link so as to be differentiated from the optical link of WDM for data service. Subscribers are therefore burdened by additional construction cost and continuous investment from the viewpoint of maintenance and operation.
The broadcasting service provision method using a DFB laser array, carrying some of the same disadvantages as the broadcasting service provision using a DFB laser, electrically multiplexes data service signals and broadcasting service signals of differing frequency bands, modulates directly each distributed feedback laser in accordance with the multiplexed signals, and then outputs the signals through optical link of WDM to provide the broadcasting service to each subscriber terminal. Similar to the case of the broadcasting service provision method using a DFB laser, this method complicates manufacturing and requires the use of high-priced elements which are needed to provide accurate wavelength selectivity and wavelength stability of the WDM optical source. Also characteristic of this method is degradation of data service signals and broadcasting service signals due to their simultaneous provision through one channel.
The broadcasting service provision method using a spectrum-sliced light source modulates directly or indirectly an optical source outputting optical signals of wide bandwidth in accordance with broadcasting service signals, spectrally slices the modulated signals, and outputs plenty of wavelength-sliced channels generated as the result through optical link of the WDM to provide the broadcasting service to each subscriber terminal. This method therefore doesn't need an optical source with specific generation wavelength and a wavelength-stabilizing circuit for stabilizing the wavelength. Examples of an optical source for the spectrum-sliced method are a light emitting diode (LED), a super luminescent diode (SLD) and a fiber amplifier light source.
Disadvantageously, transmission performance may be degraded for the broadcasting service provision method using a spectrum-sliced light source, because this method causes some distortion of the broadcasting service signals by chromatic dispersion effect. The receive sensitivity may also be degraded, because signal-to-signal beat noise generated in an optical receiver exists in the bandwidth of the broadcasting service signals. Although the LED and the SLD have extremely wide optical bandwidth and may cut the construction cost, narrow modulation bandwidth causes the transmissible capacity of the broadcasting service signals to be small, and the low output of optical sources require the addition of an optical amplifier for compensating the loss generated by the spectrum slicing. Another optical source and yet another optical amplifier must be additionally included so as to provide more capacity for broadcasting service signals. Also, although the fiber amplifier light source may provide high power for spectrum-sliced channels, use of the light source entails the high price of an external modulator.